-- union case: all subqueries are a map-only jobs, 3 way union, same input for all sub-queries, followed by filesink

explain 
  select unionsrc.key, unionsrc.value FROM (select s1.key as key, s1.value as value from src s1 UNION  ALL  
                                            select s2.key as key, s2.value as value from src s2 UNION  ALL  
                                            select s3.key as key, s3.value as value from src s3) unionsrc;

select unionsrc.key, unionsrc.value FROM (select s1.key as key, s1.value as value from src s1 UNION  ALL  
                                          select s2.key as key, s2.value as value from src s2 UNION  ALL  
                                          select s3.key as key, s3.value as value from src s3) unionsrc;
